The present invention relates to an electrostatic chuck, and a method of and an apparatus for processing a sample using the chuck. The invention relates particularly to an electrostatic chuck suitable for electrostatically holding a sheet-like sample, such as a semiconductor substrate or a liquid crystal substrate, when the sample is processed or carried, and a method of and an apparatus for processing a sample using the chuck.
An example of a bipolar type electrostatic chuck using a pair of electrodes having different polarities, in the form of an electrostatically attracting apparatus having a pair of semi-circular or concentrically-circular shaped flat electrodes, is disclosed in Japanese Patent Laid-open No. 64950/1982. According to this document, by increasing the ratio between the area of the electrostatically attracting apparatus and the electrode area of the pair of flat electrodes, mounting a substance on the pair of flat electrodes through an insulator having a thickness of 50 to 200 .mu.m, and electrostatically attracting the substance by applying a voltage between the flat electrodes, it is possible to employ the electrostatically attracting apparatus to hold both a conductive substance and a conductive substance whose surface is covered with a thin insulating film to provide a stronger attracting force and to simplify the structure of the apparatus. The document also describes that the attracting force is maximized when the areas of the pair of positive and negative electrodes are equal to each to other. Also, another bipolar type electrostatic chuck is disclosed in Japanese Patent Laid-open No. 120329/1994.
The method of holding a sample, for example, a wafer using such an electrostatic chuck has advantages in that: (1) since the surface of a wafer to be processed is not mechanically contacted by the chuck, the wafer can be prevented from being contaminated by abrasive particles and the like; and (2) since the entire back surface of a wafer is fixedly attracted on the chuck, the camber of the wafer can be corrected, so that the contact of the wafer with an attracting surface of the chuck becomes more reliable when the wafer is finely processed by etching or the like, to improve the thermal conductivity of the wafer, thereby facilitating the temperature control of the wafer. For these reasons, the electrostatic chuck is being extensively used as a sample stage (referred to simply as an "electrode") of a dry etching apparatus or a plasma processing apparatus, such as a CVD apparatus.
A bipolar type electrostatic chuck used for a plasma processing apparatus, in the form of an electrostatically attracting apparatus, for example, in Japanese Patent Publication No. 44747/1982. The document indicates that a larger attracting force can be obtained during plasma discharge by making the area of the positive electrode larger than that of the negative electrode, and that the attracting force in the case of generation of no plasma is maximized when the ratio between the areas of the positive and negative electrodes is set at 1.
Another disadvantage of the electrostatic chuck will be described below. In general, to remove a wafer from the electrostatic chuck after termination of the processing of the wafer, bar-like supports (generally, called "pushers" or "lift pins") are lifted or pushed up from the interior of the electrostatic chuck for pushing up the wafer therefrom. The mechanism involving such bar-like supports is known, for example, from U.S. Pat. No. 4,565,601 or Japanese Patent Laid-open No. 252253/1994. However, in the case where there exists a residual attracting force between an electrostatic chuck and a wafer, if a wafer is forcibly peeled from the electrostatic chuck by applying a strong force against the residual attracting force, there arises a problem that the wafer may be cracked or undergo an abnormal discharge sufficiently large to destroy devices of the wafer.
To cope with the disadvantages due to a residual attracting force, various methods for eliminating an electric charge stored on an electrostatic chuck have been proposed. For example, a method of eliminating an electric charge stored on an electrostatic chuck upon removal of a sample from the chuck is described in U.S. Pat. No. 5,117,121, wherein a residual attracting force eliminating voltage, having a polarity reversed with respect to that of an attracting voltage and which is higher than the attracting voltage, is applied between the electrodes of the chuck. Another method of eliminating an electric charge stored on an electrostatic chuck is described in Japanese Patent Laid-open No. 185773/1983, wherein a DC voltage for generating an electrostatic attracting force is turned off, and thereafter the radio frequency power for generating a plasma is turned off. Besides these, various methods of removing a sample from an electrostatic chuck are described, for example, in Japanese Patent Laid-open Nos. 112745/1989 and 247639/1992.